FIG. 1 depicts a schematic diagram of a portion of a typical wireless telecommunications system in the prior art, which system provides wireless telecommunications service to a number of wireless terminals (e.g., wireless terminals 101-1 through 101-3) that are situated within a geographic region. The heart of a typical wireless telecommunications system is a wireless switching center (e.g., Wireless Switching Center 120), which may also be known as a Mobile Switching Center ("MSC") or Mobile Telephone Switching Office ("MTSO"). Typically, Wireless Switching Center 120 is connected via wirelines (e.g., wirelines 102-1 through 102-5) to a plurality of base stations (e.g., base stations 103-1 through 103-5) that are dispersed throughout the geographic area serviced by the system and to local and long-distance telephone and data networks (e.g., local-office 130, local-office 138 and toll-office 140). Wireless Switching Center 120 is responsible for, among other things, establishing and maintaining calls between wireless terminals and between a wireless terminal and a wireline terminal (e.g., wireline terminal 150), which is connected to the system via the local and/or long-distance networks.
The geographic area serviced by a wireless telecommunications system is partitioned into a number of spatially distinct areas called "cells."0 As depicted in FIG. 1, each cell is schematically represented by one hexagon in a honeycomb pattern; in practice, however, each cell has an irregular shape that depends on the topography of the terrain surrounding the cell. Typically, each cell contains a base station, which comprises the radios and antennas that the base station uses to communicate with the wireless terminals in that cell and also comprises the transmission equipment that the base station uses to communicate with Wireless Switching Center 120.
For example, when wireless terminal 101-1 desires to communicate with wireless terminal 101-2, wireless terminal 101-1 transmits the information-bearing signals to base station 103-1, which relays the signals to Wireless Switching Center 120 via wireline 102-1. Upon receipt of the signals, and with the knowledge that it is intended for wireless terminal 101-2, Wireless Switching Center 120 then returns the signals back to base station 103-1, which relays the signals, via radio, to wireless terminal 101-2.
When wireless terminal 101-1 transmits a signal to base station 103-1, two factors dominate the determination of how much power wireless terminal 101-1 uses to transmit the signal. The first factor pertains to the quality of the received signal and the second factor pertains to the amount of interference caused by the transmission of the signal.
With regard to the first factor, the quality of the signal as received by base station 103-1 is highly correlated to: (1) the amount of power used to transmit the signal and (2) the environmental factors affecting the signal. For example, if wireless terminal 101-1 transmits the signal with too little power, then the risk exists that the quality of the received signal will be unacceptable. When the quality of the received signal is unacceptable, base station 103-1 is unable to process the signal and there is effectively no communication. As the amount of power that wireless terminal 101-1 uses to transmit increases, the signal quality also increases, albeit with diminishing returns.
FIG. 2 depicts a graph that illustrates the relationship between the received signal quality of a signal as a function of the amount of power used to transmit the signal. As is well-known in the prior art, the signal quality can be measured in accordance with a variety of well-known criteria (e.g., signal-to-noise ratio, signal-to-interference ratio, frame error rate, bit error rate, etc.). Furthermore, the amount of power used to transmit a signal can be measured in accordance with a variety of well-known criteria (e.g., absolute power as measured in dBm, average power as measured in dBm, etc.).
Clearly, the first factor mandates that wireless terminal 101-1 transmit each signal at at least the minimum power level; otherwise the signal cannot be processed and the utility of the system is undermined. Furthermore, the first factor suggests that wireless terminal 101-1 transmit each signal with substantially more power than the minimum to provide a margin of safety.
With regard to the second factor, the extent to which wireless terminal 101-1 interferes with the signals of other wireless terminals (e.g., wireless terminals 101-2 and 101-3, etc.) is highly correlated to the amount of power used by wireless terminal 101-1 to transmit its signals. For example, if wireless terminal 101-1 transmits the signal with too much power, then the signals from wireless terminals 101-2 and 101-3 cannot be received with acceptable signal quality. Therefore, the confluence of the two factors suggests that wireless terminal 101-1 should transmit its signals with as much power as necessary to ensure that its signal is received with satisfactory quality, but no more.
FIG. 3 depicts a graph of the interference caused by a wireless terminal as a function of the amount of power used by that wireless terminal to transmit its signals. As is well-known in the prior art, the interference can be measured in a variety of well-known criteria (e.g., signal-to-noise ratio, signal-to-interference ratio, frame error rate, bit error rate, etc.).
In summary, the two factors for determining the amount of power used for transmitting signals oppose each other and a balance must be maintained at all times at each wireless terminal to ensure that its signals are received with satisfactory quality yet do not unnecessarily interfere with any other wireless terminals.
A first technique in the prior art for maintaining that balance is based on: (1) the fact that the quality of the signal is highly correlated to the amount of power used to transmit the signal, and (2) the fact that the signal quality must remain at or above some minimum for the system to have any utility. In accordance with the first technique, base station 103-1 continually measures the signal quality of the signals transmitted by wireless terminal 101-1 and compares the measured quality against a target quality, called the SIR Target. If the measured quality for the signal is below the SIR Target, the base station sends a message to the wireless terminal directing it to transmit its next signal at an increased power level. In contrast, if the measured quality for the signal is at or above the SIR Target, the base station sends a message to the wireless terminal directing it to transmit its next signal at an decreased power level.
The operation of the first technique is described in detail in the flowchart of FIG. 4. The first technique begins at step 401 at which base station 103-1 establishes a minimum acceptable level of signal quality for the signals received from wireless terminal 101-1. This minimum is called the SIR Target.
At step 402, base station 103-1 receives a signal, S.sub.i-1 from wireless terminal 101-1 and compares the quality of the signal against the SIR Target. If at step 403 the measured signal quality is below the SIR Target, control passes to step 404 and a power control signal b.sub.i is set to +1, which will direct wireless terminal 101-1 to transmit its next signal at an increased power level equal to the old power level, P.sub.i-1, plus a step size, Q. Alternatively, control passes to step 405 and the power control signal b.sub.i is set to -1, which will direct wireless terminal 101-1 to transmit its next signal at a decreased power level equal to the old power level, P.sub.i-1, minus the step size, Q.
At step 406, base station 103-1 transmits the power control signal b.sub.i to wireless terminal 101-1, and at step 407, wireless terminal 101-1 receives the power control signal b.sub.i transmitted at step 406.
At step 408, wireless terminal determines the power level, P.sub.i, at which signal, S.sub.i, is to be transmitted. Typically, the power level, P.sub.i, is based on the previous power level, P.sub.i-1, the power control signal, b.sub.i, received at step 407, and the step size Q. In particular, EQU P.sub.i =P.sub.i-1 +Q*b.sub.i
From step 408, control passes to step 409, at which the signal, S.sub.i, is transmitted at the power level P.sub.i.
The first technique is disadvantageous, however, in that a single, fixed step size is often too coarse for situations in which the environmental factors affecting the propagation of the signal vary slowly. This is the rule rather than the exception for telecommunications systems in which wireless terminal 101-1 is immobile. The problem with coarse step size adjustments in situations such as this is that the power level of successively transmitted signals vacillates wildly between too much power and too little power.
A second technique exists in the prior art that attempts to ameliorate the vacillating power level of successively transmitted signals. In this technique, which is an extension of the first technique, wireless terminal 101-1 is capable of setting its step size to one of three values (e.g., 1.0 dB, 0.5 dB and 0.25 dB). Consequently, wireless terminal 101-1 can adjust its power level by 1.0 dB when the environmental factors are changing quickly, 0.5 dB when the environmental factors are changing less quickly, and 0.25 when the environmental factors are changing slowly.
The disadvantage of the second technique is that base station 103-1 only transmits one bit per power control signal, and, therefore, there is no mechanism for quickly and efficiently directing when wireless terminal 101-1 should use a given step size. The proposed solution to this deficiency is to enable base station 103-1 to send a special message, called a power control parameter update message ("PCPUM"), to wireless terminal 101-1 periodically or sporadically to set the step size that wireless terminal 101-1 uses at any given time. This solution is disadvantageous, however, in that it is too slow for situations in which the environmental factors vacillate quickly between turmoil and quiescence.
Therefore, the need exists for a technique for controlling the power level at which a wireless terminal transmits that is responsive to situations in which the environmental factors vacillate quickly between turmoil and quiescence.